Or low visibility at night
Even today, electronic systems can see better in challenging vision situations than humans - they only time the human Mark 1 eyeball is superior is in near perfect conditions (and that advantage is shrinking fast). Add some smoke or fog, low light, etc. and electronics become superior (with the advantage of being able to instantly determine distances to millimeters - far more accurate than any human). A CAT III autoland wouldn't be remotely possible using only human vision.
If we can design a car to drive down a busy, uncontrolled neighborhood road autonomously, taxing an aircraft at a controlled airport becomes almost trivial.